#8 overloading operator+ for RooCmdArg

[Gerhard Raven]

In order to trivially avoid the artificial imposed max
number of arguments
for functions taking RooCmdArg, without forcing users
to explicitly make
RooLinkedList of RooCmdArg, one could overload operator
+ for RooCmdArg to either return a RooLinkedList or a
RooCmdArg which
is a compound of other arguments. (the former requires
also to add operator+ and operator+= to
RooLinkedList). This would allow one to replace:
pdf->fitTo(*data,Strategy(0),Hesse(false),Minos(false),Save(true),NumCPU(2));
with:
pdf->fitTo(*data,Strategy(0)+Hesse(false)+Minos(false)+Save(true)+NumCPU(2));

It would also make it easier to re-use (sub)sets of
arguments for later use, and pass them around...